Optical interferometers are used in optical receivers to receive optical communications signals, such as Differential Phase Shift Keyed (DPSK) modulation signals, which carries differentially encoded data on the phase of an optical signal in time. DPSK provides approximately 3 dB sensitivity improvement over commonly used intensity modulation (IM) formats, such as On Off Keying (OOK). It also can operate with a 100% duty-cycle, in contrast to the IM formats which typically have a maximum 50% duty-cycle. Therefore, DPSK can transmit the same average power with one-half the power peak power of OOK, which can reduce nonlinear effects that can often limit the capacity of fiber-optic links. Such performance benefits come at the cost of increased complexity in the receiver, which requires a delay-line interferometer.
For optical communication systems, the interferometer is often polarization sensitive, requiring control of (i) incoming polarization and (ii) relative polarization and phase between the arms of the interferometer. Typical DPSK systems encode the binary digital data by sending bits with either a 0 or π differential phase shift, where no phase shift maps to a logical “zero” and the π phase shift can map to a logical “one.” The π phase shift corresponds to a half wavelength (λ/2) delay, which, for 1.5 μm wavelength optical signals, corresponds to about a 500 nm shift in distance in fiber or, equivalently, 2.5 fs in time. The interfering bits are usually adjacent to each other, so that the delay between them, τ, is the same as the bit period, and dependent on the data rate. For example, for data rate R=40 Gbit/s communications, τ=1/R=25 psec, or approximately 10,000 half-wavelengths; for data rate R=10 Gbit/s communications, τ=1/R=100 psec, or approximately 40,000 half-wavelengths.
In order to demodulate optical DPSK signals, the differentially encoded bits are interfered with each other. This is typically achieved using the Mach-Zehnder type delay-line interferometer, where the delay is the time duration τ between the differentially encoded bits. In order maximize the interference, the two interfering bits must have substantially the same polarization, and the delay and the differential delay must be stable to small fractions of a wavelength (e.g., <λ/10=˜100 nm in fiber or 0.5 fs). Due to these challenging constraints, existing delay-line interferometers often incorporate a microscopic phase control element that can impart fractional wavelength delays on the optical path by heating or stretching the fiber/waveguide, which, in conjunction with feedback, can be used to stabilize the interferometer. In addition, existing delay-line interferometers often make use of polarization maintaining elements or active polarization control, and minimize the use standard single-mode elements in order to improve stability.